Forex Math

Hey Y'all... I recently started trading forex again, and I used to just trade the eurusd but now I am trading more pairs because I am getting better spreads. Can someone please explain to me how to come up with the correct order sizes? I want to go from one pair to the next and have the same change in P/L per tick accross the different currencies, in USD. The thing is, the P/L is in the currency, and it throws me off. I noticed if I do a yen cross the P/L goes up much more, but then in USD it is the same amount. IB does not make this easy because you can't make set the preset order to be worth a certain USD, so I have to figure out how much units. Unfortunately, I don't quite know how it works. Thanks. -Steps